This disclosure concerns an invention relating generally to pressure plate extractors for soil testing, and more specifically to pressure plate extractors intended for leak-free operation.
The soil water characteristic curve (SWCC), a parameter which relates suction (matric, total, or both) to water content or saturation, is essential for characterizing the hydraulic and mechanical behavior of unsaturated soils. The method used to measure the SWCC depends on the texture of the soil (coarse vs. fine) and the magnitude of the suctions that must be established. For finer textured soils (silts, clays, and silty or clayey sands), a pressure plate extractor is normally used. A pressure plate extractor generally includes two key components, a pressure chamber (also referred to as a pressure cell) which allows pressurization of its interior, and a porous drain plate which rests within the pressure chamber in communication with soil to be tested, and which receives water or other liquids from the soil during pressurization. The drain plate is usually a ceramic disk, although polymeric membranes are used when very high suctions ( greater than 1500 kPa or 150 m of water) are being applied. The structure and operation of pressure plate extractors is better understood with review of common configurations of prior extractors.
FIG. 1 illustrates an exemplary pressure plate extractor 100 (commonly referred to as a xe2x80x9cTempe cellxe2x80x9d) used for applications where lower suctions ( less than 100 kPa or 10 m of water) are to be applied. The pressure chamber is defined by a lid 102, a base 104, and a cylindrical sidewall 106 (wherein the lid 102 and base 104 are also provided in cylindrical forms between which the sidewall 106 may be fit). A porous drain plate 108 is provided on the base 104 to receive water or other liquid from a soil sample provided atop the drain plate 108 in a retaining ring 110. The base 104 has a recess 112 wherein the liquid may be received. A pressure inlet 114 is provided in the lid 102 for connection to a compressed air cylinder or other pressure source, and a drain outlet 116 is provided in the base 104 to receive water or other liquid expelled from the soil sample into the drain plate 108 during pressurization. O-ring seals 118 are provided between the pressure chamber sidewall 106 and the lid 102 and base 104, and also between the drain plate 108 and base 104. A nut-screw arrangement 120 is provided whereby the lid 102 may be urged against the sidewall 106, which in turn urges against the drain plate 108 and base 104, to close the pressure chamber for pressurization.
When testing at higher pressures is desired, a pressure plate extractor having a more robust pressure chamber is generally used, with an exemplary arrangement being illustrated in FIG. 2. Here, the pressure plate extractor 200 has a pressure chamber defined by a lid 202 and a combined base and cylindrical sidewall 204. A porous drain plate 206 receives water or other liquid from a soil sample provided in a retaining ring 208. A metal screen 210 is situated at the bottom of the drain plate 206, and the screen 210 and the bottom of the drain plate 206 are then enclosed (with the screen 210 held to the bottom of the drain plate 206) by a rubber membrane 212 which is clamped about the edges of the drain plate 206 by a wire wrapping 214. A drain outlet tube 216 then extends from the exterior of the sidewall 204 to the space between the bottom of the drain plate 206 and the rubber membrane 212. A pressure inlet 218 extends through the sidewall 204, and O-ring seals 220 are provided between the lid 202 and sidewall 204 to deter depressurization of the pressure chamber. A nut-screw arrangement 222 is provided to urge the lid 202 against the sidewall 204 to close the pressure chamber for pressurization.
When using the foregoing extractors 100 and 200, the air pressure inside the pressure chamber is elevated via pressure inlets 114 and 218, and atmospheric pressure is generally maintained at the drain outlets 116 and 216 (and thus on the sides of the drain plates 108 and 206 in fluid communication with the drain outlets 116 and 216). Drying SWCC can be measured by first saturating the soil sample, and then applying a series of different pressure differentials (often referred to as xe2x80x9csuctions,xe2x80x9d since water is pulled from the soil sample owing to lower pressure at the drain outlets 116/216) between pressure inlets 114/218 and drain outlet 116/216. Different amounts of water are expelled at different pressure differentials, and the expelled water is measured (gravimetrically or volumetrically) at each suction to define the SWCC.
Although the operating principles of the pressure plate extractors 100 and 200 are conceptually simple, mechanical problems are common, with air leakage being a particular problem. Leakage is highly undesirable because it can invalidate the test results, and since a test to determine SWCC of a sample can take from two weeks to several months to run, an invalid test run can result in significant loss of time and money (and can significantly delay projects wherein the SWCC is needed to proceed). In extractors such as extractor 100, leakage is most prevalent at the outer edge or the bottom of the drain plate 108 from air bypassing the adjacent O-ring seal 118. A common solution is to glue the drain plate 108 in place on the base 104 using epoxy or another adhesive applied around the edge of the drain plate 108, but because the adhesive bond is permanent, the drain plate 108 usually cannot be removed for later cleaning, test preparation, etc. without damage. Also, the rigid connection caused by the epoxy between the drain plate 108 and the base 104 can lead to cracking of the drain plate 108 owing to the pressure differential between the recess 112 and the interior of the pressure chamber, and owing to loading of the drain plate 108 by the sidewall 106 when the sidewall 106 is urged towards the base 104 to seal the pressure chamber. These problems lead to an unfortunate tradeoff: the lid 102 must be tightly clamped to the base 104 to deter leaks, but this is more likely to crack the drain plate 108 (and conversely, air leaks may result if stress on the drain plate 108 is relieved in order to avoid damage). As a result, some degree of leakage always occurs and must be tolerated, though it degrades the quality of the SWCC test results.
The extractor 200 encounters similar problems in that air leakage occurs between the drain plate 206 and the rubber membrane 212 owing to poor sealing by the wire wrapping 214 or other sealing arrangement. Decreases in test accuracy from leakage of the extractor 200 are particularly unfortunate since test data from the extractor 200 are inherently not as precise as for the extractor 100, owing to the relatively small size of the soil sample used in the extractor 200, and also owing to inefficiencies in collecting expelled water in the extractor 200. These collection inefficiencies primarily arise from difficulties in collecting all water from the screen 201 and membrane 212, and air diffusion through the drain plate 206 interfering with measurements.
Additionally, both of the extractors 100 and 200 depicted in FIGS. 1 and 2 have limited sealing capacity between their lids, sidewalls, bases, and drain plates, since their seals 118/220 are set within recesses and can only be compressed to a limited extent. If the seals 118/220 grow less flexible over time (as is common), they may fail to provide the necessary degree of sealing regardless of how far their lids and sidewalls are urged towards their bases.
Owing to the importance of accurate SWCC measurements to civil and environmental engineering projects, and the cost and time involved in obtaining accurate SWCC measurements, there is a substantial need for improvements in pressure plate extractor apparata which overcome the foregoing problems.
The invention involves a pressure plate extractor which is intended to at least partially solve some of the aforementioned problems. To give the reader a basic understanding of some of the advantageous features of the invention, following is a brief summary of preferred versions of the extractor. As this is merely a summary, it should be understood that more details regarding the preferred versions may be found in the Detailed Description set forth elsewhere in this document. The claims set forth at the end of this document then define the various versions of the invention in which exclusive rights are secured.
A preferred version of a pressure plate extractor constructed in accordance with the invention includes a pressure chamber defined within a pressure chamber base and pressure chamber sidewalls (which may have a pressure chamber lid separately or integrally provided thereon). A drain plate sized to fit on the pressure chamber base is provided within the pressure chamber. A pressure inlet is provided, preferably on the pressure chamber sidewalls and/or pressure chamber lid, to allow pressurization of the pressure chamber. Similarly, a drain outlet for receiving expelled water or other liquid from the drain plate is provided on the pressure chamber base. The drain plate has opposing plate inner and outer faces bounded by a plate intermediate edge, with the plate inner face being situated adjacent the interior of the pressure chamber and the plate outer face being situated outside the pressure chamber interior. The drain plate preferably rests within a depression defined in the pressure chamber base, with the plate intermediate edge being spaced inwardly from the outer walls of the depression.
The pressure chamber sidewalls are preferably sized to extend about the entirety of the drain plate""s perimeter, as opposed to being sized to fit atop the drain plate as in the prior pressure plate extractors shown in FIGS. 1 and 2. Thus, if the pressure chamber sidewalls are urged towards the pressure chamber base, they need not bear against the drain plate and stress it, as in the prior pressure plate extractors.
A sealing arrangement is then provided which is believed to offer significant advantages over the prior pressure plate extractor arrangements of FIGS. 1 and 2. A seal, which is preferably formed of an elastomeric strip or ring, is fit about the intermediate edge of the drain plate, and between the drain plate""s intermediate edge and the outer walls of the depression formed in the pressure chamber base. The pressure chamber sidewalls are then fit atop the seal between the drain plate and the depression outer walls, and they bear downwardly against the seal to press the seal against the pressure chamber base. This deforms the seal, causing it to expand laterally to tightly engage the drain plate and depression outer walls in the pressure chamber base. As a result, the seal is engaged between all of the pressure chamber sidewalls, the drain plate, and the pressure chamber base. The greater the force used to urge the pressure chamber sidewalls toward the pressure chamber base, the tighter the seal between the sidewalls and base (and between the sidewalls and drain plate), and the tighter the resulting seal between the drain plate and the pressure chamber base. At the same time, the pressure chamber sidewalls do not bear against the drain plate, thereby diminishing the likelihood that the drain plate will fracture. A substantially leak-free pressure chamber with low probability of drain plate failure results.
Advantageously, a pressure plate extractor of this nature is suitable for use at high pressures as well as low pressures, and thus can serve as a replacement for both of the extractors depicted in FIGS. 1 and 2. It can provide substantially higher measurement accuracy than the prior high-pressure extractor arrangements because it does not require use of an inefficient mesh-and-membrane arrangement to collect expelled liquids.
Further advantages, features, and objects of the invention will be apparent from the following detailed description of the invention in conjunction with the associated drawings.